1. Field of the Invention
The present invention relates to a printed matter inspection device, a printing press, and a printed matter inspection method.
This application is based on Japanese Patent. Application No. 2006-327549, the content of which is incorporated herein by reference.
2. Description of Related Art
In general, an inspection is required for a printed matter printed with a printing press so as to check the printing quality, such as stain, misregistration, and excess or insufficiency in printing density of the printed matter. A printing press usually has an inspection device for such an inspection. The inspection device includes an image reader that reads an image of a pattern on a printed matter. The inspection device inspects the printed matter on the basis of the image read with the image reader (for example, see the Publication of Japanese Patent. No. 3790490).
For reading the image, the inspection device irradiates a printed matter with light, and detects reflected light reflected by the printed matter. A popular image reader for a color printed matter employs, for example, a typical light source or a plurality of light emitting diodes (LEDs) as a device for irradiating a printed matter with light (for example, see the Publication of Japanese Patent. No. 3801571).
Unfortunately, the quantity of light emitted from the LEDs may change (drift) with time, and hence, using the LEDs as the device for emitting illuminating light may result in an inaccurate inspection for the printing density or the like due to the time-lapse change in the quantity of light.
Owing to this, in the inspection device with the LEDs, it is necessary to measure the time-lapse change in the quantity of light emitted from the LEDs and to compensate the change in the quantity of light.
As a method of measuring the time-lapse change in the quantity of light emitted from the LEDs, a method is known that LEDs irradiate a non-print area, or a blank area on a printed matter with light and an inspection device detects the intensity of light reflected by the blank area so as to measure a time-lapse change in the quantity of light emitted from the LEDs. That is, a method is for detecting a blank level.
A printed matter usually has a non-print area between print areas. The method using the non-print area as a blank area is popular.
Such a non-print area tends to be narrowed because it is desired to reduce the cost of print sheets and to increase print areas.
A non-print area is an area with no pattern printed, however, the non-print area may not be always blank due to a stain or the like. This may further narrow an area for the measurement of the blank level in the non-print area.
The blank level of a color printed matter is measured by irradiating the color printed matter with a plurality of different color light beams while the color printed matter being conveyed, and time-dividing the quantity of each of reflected color light beams.
If the area for the measurement of the non-print area is narrowed, then an area (measurement area) used for the measurement of the quantity of one of the reflected color light beams may extend from the non-print area to the print area.
It is difficult to accurately measure the time-lapse change (blank level) in the quantity of light emitted from the LEDs as long as the measurement area contains the print area.
To solve the above-mentioned problem, a method is conceivable that reduces the period of acquiring an image during the detection of the blank level, and increases the number of inspection lines. Increasing the number of inspection lines, the measurement area with the color light beams may be located within the non-print area, so that the blank level can be accurately measured.
However, in order to increase the number of inspection lines, the clock frequency of a detector has to be heightened to correspond to the increase in the number of inspection lines.
An existing element corresponding to a low clock frequency does not correspond to a high clock frequency. It is necessary to entirely change components of the detector including a control circuit, thereby increasing the cost.
As the period of acquiring an image is reduced, an existing data transmission system for transmitting image data may not reliably transmit the image data because the data transmission rate is insufficient.
To solve this, a method is conceivable that reconfigures the data transmission system, however, the cost may be increased.
Also, time is necessary for stabilizing the output of a light-detecting amplifier provided in the detector, and hence, there is a limit for reducing the period of acquiring the image, resulting in a limit for increasing the number of inspection lines.